Hepatitis C virus (HCV) infects approximately four million Americans and at least 200 million of the world's population. Most patients (approximately 85%) go on to develop chronic infections that over the course of two to three decades can cause hepatitis, cirrhosis, hepatocellular carcinoma, and immune complex disease. A significant proportion will develop end stage liver disease, which in the US, is the leading indicator for liver transplantation. Deaths due to HCV are estimated at 10,000 yearly; the CDC expects such to quadruple over the next decade. HCV therapy is limited to alpha-interferon and ribivirin and is only successful in a minority of patients. Other methods of intervention, including especially vaccination, are sorely needed. During the previous grant period, we have characterized responses to HCV antigens at the epitope level. One of our most intriguing observations is that despite the fact that many HCV epitopes stimulate strong production of IL-10, a few epitopes stimulate robust production of IL-2 and IFN-gamma, which should promote viral clearance. In the context of the intact viral antigen, however, these so called Th1 responses are suppressed even though the epitopes are processed appropriately by APC. It is our hypothesis that by focusing immune responses on such "cryptic" Th1 epitopes, it may be possible to ameliorate the consequences of HCV infection or even eliminate virus altogether. This proposal seeks to further understand the basis for viral escape and to develop an in vivo model in which to test vaccination schemes aimed at focusing immune responses towards specific Th1 epitopes of HCV. The proposed studies set the stage, and indeed comprise a necessary prerequisite, for the development of a therapeutic vaccine for HCV, which could ultimately cure chronic infection. Of course, along the way, we will learn much about how this important human pathogen has adapted to exploit and regulate the immune response, features of which can most certainly be applied to other aspects of human immune dysfunction. We propose to determine how epitope specific regulation occurs in responses to HCV antigens in vitro; to develop and characterize an HLA transgenic model in which to study epitope specific responses to HCV antigens in vivo; to devise and evaluate vaccination strategies to counter epitope-specific negative regulation and to focus anti-viral responses towards Th1 epitopes.